Ruminants such as cattle, sheep and goats can feed upon and digest cellulose obtained from a variety of plant sources such as grasses, due to the presence in the rumen of the animal of cellulolytic enzymes. However, the ability of the cellulolytic enzymes to degrade cellulose varies considerably according to the nature and origin of the cellulose containing substrate. Ruminants cannot digest ligno-cellulose materials such as untreated wood, for example. Untreated wood contains lignin, hemicelluloses and cellulose. When lignin is removed, the remaining cellulose product is readily and completely digestible by the ruminant. Considerable effort has been expended in the past by animal nutritionists and others to devise economical ways in which low grade forage such as wood can be rendered digestible by ruminants.
The reasons for the indigestibility of ligno-cellulose materials, such as wood, by ruminants are not fully understood, and do not appear to be based upon any single or simple factor. Complete or partial removal of lignin from the ligno-cellulose material is not necessary. Lignin itself is not a toxic or harmful substance for the ruminant to eat. It appears to be of no nutritional value to the animal. If eaten, it passes through and is excreted by the animal in substantially unchanged form. Delignification of wood so as to provide cellulosic animal fodder is an uneconomic proposition, firstly because of the energy and chemicals needed in the delignification process itself, and secondly because of the problems of the disposal of the essentially useless and potentially pollutant lignin materials so obtained. It has been suggested that the physical or chemical form of the cellulose when in association with lignin in a ligno-cellulose material is responsible for its indigestibility by ruminants. Thus, the cellulolytic enzymes may have difficulty gaining the necessary access to the cellulose molecules to degrade them chemically, in ligno-cellulose material. This could be as a result of the lignins forming a physical barrier to access to the cellulose molecules. Alternatively, it could be that the cellulose assumes a condition of increased crystallinity, or decreased surface area, or increased molecular chain length when in association with lignin in a ligno-cellulose material, any of which conditions could inhibit the cellulolytic enzyme attack on the cellulose for digestion by the ruminant.
There is a great incentive to provide ruminant fodder derived from ligno-cellulose material, due to the abundant availability of ligno-cellulose material, much of which is not useful for other economical purposes. However, it is very important that any process for rendering ligno-cellulose material suitable for animal fodder be economical to operate on a commercial scale, since the end product must presently compete with other low cost aminal feed products.